Terrarium substrates. Which one to choose?
Tropical, forest, savanna substrate, clay, drainage and quartz sand — what each one is really for and how to choose the right one for your terrarium. Based on the range of the Polish brand Soilico.
It's easy to underestimate substrate — it's tempting to treat it as just "that layer on the bottom" of the terrarium and nothing more. In reality it does a lot: it holds the humidity the animal needs, it's home to the clean-up crew in a bioactive terrarium, it lets burrowing species dig, and the health and stability of the whole terrarium ecosystem depends on choosing it well. Different types of terrariums need completely different substrates — a mix made for a humid tropical forest behaves completely differently from one prepared for a dry savanna.
Below is a practical overview of the main types of substrate available from the Polish manufacturer Soilico, based on its own product descriptions and specifications.
Tropical substrate — for humid terrariums
A ready-to-use substrate designed for terrariums with higher humidity. Its main job is water retention — it holds moisture for a long time while staying airy and stable, instead of turning into a soggy, compacted mass.
This type of substrate is meant to recreate humid rainforests and lush jungles, creating natural, comfortable conditions for reptiles, amphibians and invertebrates alike. Besides just mimicking the look, it also supports the development of healthy microfauna, which translates into a more stable ecosystem inside the terrarium.
In practice it works well as a solid base in tropical terrariums, as well as a top layer in bioactive setups. Ready-made tropical substrates of this kind usually don't clump, don't produce dust, and keep a soft, natural consistency over time.
Forest substrate — a base for the clean-up crew
A natural, organic substrate designed to recreate a humid forest ecosystem. It contains ground oak leaves, which give it the character and structure of real forest litter.
This is exactly the type of substrate a bioactive "clean-up crew" needs — isopods, millipedes and other detritivores rely on an environment rich in organic matter to live and reproduce. The oak leaf content further supports the growth of beneficial microflora, helping keep the whole bioactive ecosystem stable.
It's a good base for most species of isopods, millipedes and similar detritivores — it holds moisture for a long time without losing airiness, which is a key balance in a bioactive terrarium.
Structural clay — for shaping walls and burrows
A mix of clay, sand and calcium carbonate, designed for desert and semi-desert terrariums.
This clay was created primarily for shaping walls, backgrounds and cave-like structures in the terrarium, though it also works great as a standalone substrate — once dry it keeps its given shape and structure, faithfully imitating a desert or semi-desert environment.
In use, simply mix the clay with a small amount of water, gradually adding more until you get a consistency similar to modeling clay — pliable but not too wet. This makes it easy to shape walls, caves or other structures. Once the work is done, the terrarium needs to be left in a dry place to let the clay fully dry before any animals go in. It's usually available in several color variants (brown, red, yellow and black), so it can be matched to different desert or rocky climates.
Coconut substrates — fiber and chips
Substrates made from coconut shells, available as fine fiber or chips in two grades — coarse and fine.
Coconut chips (CocoBase) come in two grades — coarse and fine — and are mainly used to improve substrate permeability: they retain moisture without becoming waterlogged, improve aeration, and resist mold and rot. They work well both as an additive to another substrate and on their own. Coconut fiber (CocoEarth) is a finer, more compact alternative — a light, organic substrate for humid terrariums that holds moisture well while staying airy.
Coconut substrates of this kind are a very common choice for, among others, snakes — precisely because the grain size can be matched to the specific species and terrarium size.
Savanna substrate — for species from dry and semi-dry areas
A substrate designed to recreate the structure and color of the semi-dry soil of the African savanna.
It holds moisture well in the deeper layers while staying drier on the surface — conditions close to what animals from dry and semi-dry regions actually experience in nature.
In use: spread the substrate evenly over the bottom of the terrarium, forming a layer at least 3 cm thick, then spray it generously with water until the whole surface is well moistened. Leave it to dry completely — during this time the top layer will harden, forming a stable structure resembling dry savanna soil. Once dry, it's ready for use. Slight crumbling of the surface after drying is completely normal for this type of substrate. Since it's intended for dry and semi-dry terrariums, it's best not to overwater it or mist it too often once it has set.
Drainage layer — the foundation of a planted terrarium
A specialized mineral drainage layer designed for planted terrariums — the first, lowest substrate layer in a tropical or humid terrarium, where properly draining excess water is crucial.
This is expanded clay with a 10–20 mm grain size, with no chemical additives — safe for plants, animals and microfauna. Its job is to capture and drain away excess water so it doesn't sit in the substrate above, which protects plant roots from rotting and helps maintain healthy conditions for the whole ecosystem. The porous structure also allows air circulation in the lower part of the terrarium and supports the natural biological filtration process.
In a typical bioactive setup, this drainage layer sits right at the bottom, separated from the substrate above by a mesh so the two layers don't mix together over time.
Quartz sand — a universal additive and desert substrate
Very fine, selected quartz sand — over 98% pure quartz (SiO₂), with a fine grain size of 0.1–0.5 mm, made in Poland.
Its very high purity means a minimal amount of contaminants, heavy metals or water-soluble substances — which makes it truly versatile. It works well as a standalone substrate in desert and steppe terrariums, giving a natural coating resembling sandy dunes, and its chemical inertness also makes it a safe substrate for aquariums. It's often used simply as an additive, mixed into heavier substrates to loosen them up — it's useful in mixes for cacti, succulents and terrarium plants.
Which substrate will actually work for a crested gecko?
Crested geckos (Correlophus ciliatus) come from the humid tropical forests of New Caledonia, so their terrarium should reflect that — an arboreal layout, high humidity, and a substrate that holds moisture without turning into a swamp. That clearly points to tropical substrate as the base. If you're running a bioactive terrarium with isopods and springtails as the clean-up crew, forest substrate makes a great addition — mixed in or used as a base layer for the crew itself. If the terrarium is heavily planted, it's worth adding a drainage layer at the very bottom regardless of the rest, to drain excess water away from the roots.
Savanna substrate, structural clay and pure quartz sand, on the other hand, are made for animals from dry and semi-dry regions — that's not what you need as the main substrate in a humid crested gecko terrarium. A small amount of quartz sand can still come in handy elsewhere though, for example as an additive to substrate used for terrarium plants.